The present invention relates to a safety system and to a method of operating a sensor in a safety system. It is known that sensors are used as signal generators in safety systems. Sensors considered within the scope of the present invention are used for sensing at least one form of material stress, such as occurs as a result of tensile and/or compressive forces during the deformation of material, and also during the transmission of structure-borne noise. Such sensors and related safety systems are used as accident signaling devices in the aerospace industry and the motor vehicle industry, among other fields.
It is an object of the present invention to provide a more reliable method of operating at least one sensor in a safety system, and to provide a corresponding safety system.
This and other objects and advantages are achieved by the safety system and method according to the invention, in which at least one sensor is provided that, in addition to its sensor characteristic, also has the characteristic of an actuator. Such a reciprocal method of operation is known in the case of several types of converters, including among others, piezo electric devices which are crystal structures that exhibit a charge separation on their surfaces under the effect of tension and pressure, such that an electric voltage can be tapped at their outside, by way of electrodes. Inversely, it is also possible to deform a piezo-electric crystal by applying an electric voltage to the outside. Accordingly, by an appropriate selection of the material and shape of the piezo-electric crystals, it is possible to convert surface deformation and structure-borne noise effects over a wider frequency range, to electric signals by way of the connected mechanical deformations of the a piezo sensor element. On the other hand, surfaces can also mechanically be caused to vibrate by electric signals by means of the same piezo sensor because of its suitability as an actuator.
In addition to the use of quartz based piezoelectric crystals, which may also be constructed as ceramic elements on a barium titanate or BaTiO3, or lead zirconium titanate or PZT basis, the use of piezo-electric plastic foils or films has also become known. BaTiO3 powder can be converted to a ceramic body with component or sensor dimensions, and with a solid shape, by sintering a compression-molded powder. In contrast, piezo-electric plastic foils are lastingly flexible, have a thickness of only approximately 5 to 500 μm, are light, and can be made into any flat shape almost arbitrarily, by a simple cutting. The plastic materials which are used for this purpose are, as a rule, highly polar substances which are heated during production of the foil, and are subjected to a very static electric field, for a uniform alignment of the molecules. In the course of cooling, this forced alignment of the molecules is virtually firmly frozen into the foil body. Thus, for example, German Patent Document DE 40 25 564 C1 suggests the use of polyvinylidenfluoride PVDF or other polymers consisting of polar molecules. In each case, the above-mentioned substances, which are considered as passive structural parts, already have a charge potential, and exhibit a measurable capacitive charge change under the effect of pressure. As a result, feeding by an external voltage source is unnecessary.
The above-mentioned materials share the common feature that the characteristics of the sensor and the actuator can be combined in one system, so that a simple self-test is possible, under the control and the subsequent analysis by a central system unit. For this purpose, the sensor element is first controlled as an actuator, and the excited mechanical vibration can again be sensed as an electric signal if the element is operating without disturbance. As a result, a sensor can therefore be checked at any time without additional equipment-related expenditures with respect to signal generators, etc., and can be monitored for its operating characteristics.
However, in addition to the self-tests, by means of the actuator operation a mechanical vibration can also be induced in a system to be secured, and the same element can be used, subsequently, to perform an analysis of the excited vibration with an examination of the characteristics of the system, which can be supplied to an analyzing unit of the actual safety system. Thus, in addition to deformations of the material, cracks and disturbances can also be detected, particularly by a deviation in the frequency pattern of the system response. A suggested method of operating a sensor with a reciprocal operating mechanism in a safety system therefore provides the advantages of a reliable self-test as well as the advantage of a safety check and system analysis which requires low expenditures and has a negligible interference influence, in a total mechanical system to be monitored, with the possibility of a diagnosis for the conclusion of each check.
In an advantageous further embodiment of the invention, a safety system comprises a plurality of sensors which are distributed over a structure to be secured, as the observation or monitoring area. In some sense, the mechanical structure to be observed is therefore covered by a network of sensors which, by the distribution and distribution density of the sensors, takes into account a respective peculiarity of the mechanical structure. Also in this arrangement, each individual sensor can also carry out a self-test by analyzing the residual vibrational behavior after an active test excitation by means of a predefined test signal and of a received measuring signal in the above-illustrated manner. In addition, it is possible to carry out a surface covering evaluation for the analysis of faults and/or disturbances, by actively exciting a sensor, and analyzing the measuring signals received from all sensors. All sensors can be constructed and fixed in the same manner and therefore can be part of one sensor series or of one type.
Furthermore, in modern vehicles, extensive resonance phenomena occur, which can be clearly perceived as noise also in a vehicle interior. In the form of mechanical steady-state vibrations and standing waves, such resonance phenomena lead to increased wear and stability problems on carrying parts, due to increased stress. One example of these frequently occurring problems are vehicle occupant compartments or cabins which were constructed using new composite materials based on carbon fibers or carbon-fiber-reinforced plastic composite materials. These light and extremely hard materials are used particularly in space flight, in aircraft construction and increasingly also in motor vehicle construction. Since carbon-fiber-reinforced plastic materials do not significantly dampen vibrations, because of their high inherent rigidity, vibrations within the moving systems are conducted away from an originating site (for example, from a wheel suspension, an engine or a turbine), toward the cabin or a vehicle occupant compartment. Particularly when used in a commercial aircraft, the noise level from a rear turbine via the occupant compartment (as a rigid tube) is still acoustically intensified by the fact that carrying parts of the floor and of the ceiling operate as sounding boards.
Over longer periods of time, such noise events may have harmful effects on health; they definitely decrease the ability to concentrate and impair the well-being of the vehicle drivers and passengers. The described phenomenon is therefore safety-relevant in two respects. The vibrations occurring in the above described example can now be detected in a selective manner by the use of a plurality of sensors according to the invention. Thus, maxima and minima of a resonance phenomenon can be determined in a distributed manner, by analysis of the received signals over a surface or another structural shape. By means of an active triggering of the actuator characteristics of the sensors according to the invention, vibrations can be damped by triggering an antiphase excitation, and ideally, resonances can be eliminated
Another difficulty that occurs, with the use of carbon-fiber-reinforced plastic materials is the provision of a durable connection between the different components of bearing and/or safety-relevant structures. In the area of metallic materials, it is known that steel types and aluminum cannot be connected by welding. But different types of steel also cannot be welded to one another, or can be welded only to a very limited degree. In addition, there are construction materials, such as the so-called multiphase steel types, which, due to their special characteristics, would be damaged considerably by the entry of heat during a welding operation.
In the areas mentioned above as examples, in addition to applications in aviation, many different adhesive joints are now increasingly found in motor vehicle construction. Sensors on a foil carrier (specifically, sensors constructed in the form of piezo-electric plastic foils) can also be used in a particularly advantageous manner for monitoring the durability and reliability of such adhesive connections. In one application form, piezo-electric sensors span and/or cover an adhesive joint in the form of a foil. In this case, the foil preferably extends from one structural component to the other structural component or components which are connected with one another by the adhesive joint. After an initial or starting calibration of the signals emitted by the sensors of the foil under normal conditions, changes of the adhesive joint due to tensile or compressive loads of individual sensors of the foil can be electrically measured. A continuous detachment of the adhesive connection can therefore be determined as well as, for example, a tearing or even tearing-off caused by an accident.
Irrespective of this application purpose, in the event of nonharmonic vibrations, such as occur for example as a result of disturbances, crashes or outside contact as well as pedestrian recognition, etc., it becomes possible to achieve precise recognition of a particular event, and to locate it by way of the respective sensor elements. Suitable countermeasures can then be triggered in a targeted manner by subsystems connected to the output side of the safety system as a whole, particularly the targeted and precisely defined triggering of seat belt tighteners, different air bag systems or other active safety components.
The method according to the invention therefore provides that robust, overload-resistant, cost-effective sensors are used which are capable of carrying out a self-diagnosis. Such sensors can be durably and reliably retrofitted, by embedding, gluing-on, screwing-on or the like at various points. For constructing a pure early-warning system for accidents, bumpers, doors, roof areas and the engine hood of a motor vehicle are particularly suitable. Advantageously, a sensor already provided for a parking distance control unit, abbreviated PDC, with a reversible characteristic in the form of a piezo element can also be used according to the invention, so that synergistic effects can be utilized.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.